1. Field of the Invention
The present invention relates to a method for estimating an absolute distance between a tracking laser interferometer and a retroreflector, the tracking laser interferometer (the interferometer in short) including; the retroreflector for reflecting and returning incident measurement light in an incident direction; and a two-axis rotating mechanism for rotationally moving in an exit direction of the measurement light so that optical axes of the measurement light and return light are collimated, which outputs a measurement value according to an increase or decrease in distance between the interferometer and the retroreflector, and the tracking laser interferometer using the method.
2. Description of Related Art
In Japanese Published Unexamined Patent Application No. S63-231286, there is disclosed a tracking laser interferometer for a moving body using a laser beam, wherein a two-axis coordinate of a moving body is measured based on triangulation by use of a two-axis rotating mechanism for controlling a laser beam and an exit direction of the laser beam and each readout mechanism for reading an angle. This device is characterized in that a distance between the interferometer and the moving body is measured by monitoring a deviation amount of an optical axis of return light (beam) from retroreflector means (corner cube) provided to the moving body and then by performing arithmetic processing of the deviation amount together with rotational angle information given to the rotational mechanism.
However, this technology has a problem such that measurement accuracy is limited by accuracy of triangulation and accuracy according to the above method since this technology has a mechanism for additionally providing a measurement value of the optical axis direction by the above method to a measuring device having a function for measuring a spatial two-axis coordinate based on the triangulation. As a result, this technology has seldom been used in the measurement field in which development for high accuracy is being made.
Furthermore, as a device for controlling an exit direction of a laser beam and significantly improving the measurement accuracy, tracking laser interferometers by laser interferometry length measurement have been put into practical use. Such tracking laser interferometers include “Laser Tracker X” manufactured by FARO Technologies Inc., “Laser Trackers LTD 709, LTD 840” manufactured by Leica Geosystems AG, etc.
These conventional tracking laser interferometers include an optical system 10, a two-axis rotating mechanism 40, and a controller 50, as shown in FIG. 1. Herein, the optical system 10 is divided into a laser interferometer measuring device 20 for measuring a distance to a retroreflector 70 fixed to a measurement object 60; and a tracking optical system 30 for use in a tracking control system.
The tracking control system directs measurement light to the retroreflector 70 so that the laser interferometer measuring device 20 continuously performs measurement. The retroreflector 70 is an optical element by which optical axes of incident light and reflecting light are collimated. The incident light and the reflecting light become point symmetric with respect to a center point of the retroreflector. Thus, when the incident light is incident at a point away from the center point of the retroreflector 70, a shift occurs in the reflecting light. The tracking optical system 30 measures this shift of the reflecting light (return light) relative to the incident light (measurement light), so as to allow the controller 50 to control the two-axis rotating mechanism 40.
FIG. 2 shows an optical system of the tracking laser interferometer. Measurement light from a laser interferometer measuring device 20 passes through a tracking optical system 30 and is reflected at a retroreflector 70, so as to return to the laser interferometer measuring device 20. The tracking optical system 30 includes a half mirror 32 and a light spot position detecting element 34 in which reflecting light (return light) from the retroreflector 70 is branched at the half mirror 32 and enters into the light spot position detecting element 34.
FIG. 3 shows an optical system in a case where the retroreflector 70 moves in a direction perpendicular to the optical axis. On the optical axis of the laser interferometer measuring device 20, a point around which the two-axis rotating mechanism 40 rotates is given as rotation center O. When the retroreflector 70 moves, laser light that has been reflected at a point P0 on the retroreflector 70 passes through an optical path OP1P2O′, therefore, the optical axes of the measurement light (incident light) A and the return light (reflecting light) B do not coincide. At this time, a laser spot monitored by the light spot position detecting element 34 moves from Q0 to Q1. Thus, by rotating the two-axis rotating mechanism 40 by a degree of ∠P0′OP1 so that the laser spot at Q1 returns to the initial position Q0, the optical axes of the measurement light A and the return light B coincide, thereby making continuous measurement possible even when the retroreflector 70 moves.
However, the conventional tracking optical systems have had the following problems.
FIG. 4 shows an optical system in the case where the retroreflector 70 moves in the optical axis direction of the laser interferometer measuring device 20. The measurement light A from the laser interferometer measuring device 20 passes through an optical path OP1P2O′ when reflected at the retroreflector 70a, whereas it passes through an optical path OP1′P2′O′ when reflected at the retroreflector 70b that approaches the laser interferometer measuring device 20. In both cases where the retroreflector is at the positions of 70a and 70b, on the light spot position detecting element 34 to be observed, a laser is spotted at the same point Q1, therefore, in both cases, a deviation output signal d becomes the same. The tracking laser interferometer controls the two-axis rotating mechanism 40 and thereby performs feed back control so that the deviation output signal disconstantly minimized. However, when an attempt is made to adjust the measurement light A to P0 and P0′ based on an output signal from the light spot position detecting element 34, a difference occurs in the rotational angle of the two-axis rotating mechanism 40 required according to a distance between the laser interferometer measuring device 20 and the retroreflector 70.
In order to perform control reflecting of the angle difference, it is necessary to reflect information about an absolute distance between the laser interferometer measuring device 20 and the retroreflector 70 on the control of the rotational angle. However, since the laser interferometer measuring device 20 measures a relative distance of a wavelength order with respect to an interferometer light source, generally, it cannot measure an absolute distance.
In order to solve this problem, a method exists for measuring an absolute distance prior to measurement by a tracking laser interferometer and, during the measurement, summing the prior measured absolute distance and the relative distance between the laser interferometer measuring device 20 and the retroreflector 70 to obtain the current absolute distance. In this case, the method for measuring the absolute distance mainly includes the following two types.    (1) To calculate an absolute distance by measuring a relative distance between a datum point and the laser interferometer measuring device, by giving a point at which the distance from the laser interferometer measuring device is known as a reference datum point.    (2) To mount a distance sensor capable of measuring an absolute distance.
The method (1) enables measurement of an absolute distance only by moving a retroreflector to the datum point. However, when the tracking is interrupted and the absolute distance is not known, it is required to return the retroreflector to the datum point to remeasure the absolute distance, thereby requiring troublesome operations.
On the other hand, with the method (2), since the absolute distance can be constantly measured, even when the tracking is interrupted, the absolute distance can be determined again instantly. However, there have been problems that, since a distance sensor needs to be separately mounted, thereby making mounting and handling of the interferometer complicated, this leads to cost increases.